1. Field of the Invention
The present invention is related to a method for switching decoupled plasma nitridation processes of different doses, particularly to a method for switching decoupled nitridation processes quickly.
2. Description of the Prior Art
In order to increase the integration of a single wafer, semiconductor elements are made smaller and more compact. However, for higher performance, the thickness of a gate oxide layer of a complementary metal oxide (CMOS) device is decreased to maintain the capacitance between a gate and a channel. This is because the bigger the capacitance, the smaller the electric field within the gate oxide layer, and while the electric field is small, current leakage is prevented. For example, in a semiconductor process beyond 130 nm, an oxide gate layer smaller than 20 angstroms is required to achieve good performance.
Generally, silicon oxide is used as a gate oxide layer. However, a thin layer of silicon oxide cannot meet the requirements of having a high dielectric constant, stable thermal properties, a high breakdown voltage, and small current leakage. For example, leakage currents may occur in silicon oxide layers with thickness smaller than 50 angstroms due to electrons and holes tunneling through the energy barrier of the silicon oxide layer. To fix this shortcoming, nitrogen is doped into the silicon oxide layer so as to increase the dielectric constant of the silicon oxide layer. As a result, a gate oxide layer with the same capacitance and larger physical thickness, i.e. a gate oxide layer with the same equivalent oxide thickness (EOT), is formed.
One way to dope a gate oxide layer with nitrogen is by a plasma nitridation process, such as a single step decoupled plasma nitridation (DPN) process. In a DPN process, a plasma nitridation process and an annealing process are performed to form an oxide layer with an EOT smaller than 11 angstroms. Generally, a complete DPN process includes an oxide deposition and a cooling process prior to the DPN doping process and a post nitridation annealing (PNA) process and a cooling process after the DPN. The DPN process not only decreases the current leakage efficiently, but also offers a better barrier to boron, so as to increase the performance of a transistor.
In different semiconductor device manufacture processes, the requirements of nitrogen concentration are different. However, those processes may be performed in the same chamber sequentially. Therefore, after a nitridation process is performed, the nitrogen concentration needs to be changed to fit the next nitridation process. Without a process to adjust the nitrogen concentration, the next nitridation process may be affected by the nitrogen concentration of the nitridation process just performed. This is called the memory effect. For example, after performing a 9% nitridation process, the nitrogen concentration of the chamber is too high for a 6% nitridation process, which is performed next. The unstable nitrogen concentration may affect the quality of the gate oxide and the stability of the semiconductor device.
As a result, between two nitridation processes with different doping parameters, several dummy wafers are inserted into the chamber for nitridation to adjust the nitrogen concentration of the chamber. To eliminate the memory effect, a nitrogen concentration adjusting process is provided in the prior art. According to the process, a dummy wafer is inserted to perform a complete DPN process, which includes an oxide deposition, a cooling process, a DPN doping process, a PNA and a cooling process following that. However, the adjusting process above is time consuming. In 90 nm processes, a nitrogen concentration adjusting process takes at least ten dummy wafers to recover the nitrogen concentration in the chamber. For example, in a 9% DPN process, about fifteen dummy wafers are needed to recover the nitrogen concentration of the chamber. It takes about one hour to finish nitridating fifteen dummy wafers. The higher nitrogen concentration of the DPN process, the more dummy wafers are needed. In a 13% DPN process, it takes about three to four hours to finish a nitrogen concentration adjusting process involving sixty to seventy dummy wafers. Therefore a time saving and effective method for adjusting the nitrogen concentration of the chamber is needed to meet manufacturing requirements.